Vehicle cooling systems typically have a coolant temperature sensor for providing coolant temperature information to the electronic engine controller and a thermostat for providing constant coolant temperature control. Coolant temperature is a very important parameter in several engine control strategies, and in particular in an electronically controlled fuel supply system. If the coolant temperature sensor is degraded, fuel consumption and emission strategy may be degraded. For example, if the coolant temperature sensor is indicating that the engine is cold, rather than warmed up, a rich fuel-air mixture may be supplied longer than necessary, thus potentially degrading emissions and fuel efficiency.
One method of diagnosing the engine coolant temperature sensor is described in U.S. Pat. No. 4,274,381. Engine coolant temperature is inferred from another temperature sensor such as the temperature sensor of the catalytic converter. This inferred value is compared to the value read by the coolant temperature sensor. If the two values are not the same, degradation is indicated. Then, a signal corresponding to the output of the engine coolant temperature sensor under normal engine operating conditions replaces the output of the degraded coolant temperature sensor.
The inventor herein has recognized a disadvantage with this approach. In particular, there is not a way to determine which one of the above mentioned sensors is degraded. Also, providing a predetermined signal to replace the degraded sensor information is not an accurate representation of the actual operating conditions, especially at high/low ambient temperatures, or at engine start-up.
Another disadvantage is that this method does not diagnose the cooling system thermostat. If the thermostat performance is degraded, efficient temperature levels will not be maintained under all operating conditions, and thus, vehicle performance, fuel efficiency and emission control may be degraded. Further, the prior art does not take into account the state of the thermostat (open or closed) when estimating coolant temperature.